One of the most important thing the field does for Earth is that it shields us from solar wind. Solar wind is a stream of charged particles that come flying off from the sun. Without this protection the surface would be not a very habitable place.
The field is responsible for the northern lights. The northern lights are commonly seen in the latitudes between 60 and 72 degrees north and south, just within the Arctic and Antarctic polar circles. The light is created by collisions of charged particles created by the magnetic field. The light is usually created in the ionosphere, about 80km (50 miles) up.